Temp indicator chimes although temp is really 100 degrees low.

The default for most Miele oven is 375 - however this is an earlier model oven that only allowed (as yo said) ten degree increments - I am afraid that there is no solution to that.
370 or 380 would be your options

Hello....Go and get yourself an oven thermometer form a store like Target.Ther are accurate from 100 to 650 degrees and can either sit on, or hang from your oven rack...Then you can be sure...I hope this was helpful....PEACE....

The thermostat and control will have to be replaced

This Conversion table is from the old Gas Mark scale
I'm thinking it should be right or at least pretty close.
You could also get a cheap ($5-7) oven thermometer from local grocery store.
Put it inside oven to check actual temp achieved.

Gas mark Fahrenheit Celsius
1⁄4 225° 107°
1⁄2 250° 121°
1 275° 135°
2 300° 149°
3 325° 163°
4 350° 177°
5 375° 191°
6 400° 204°
7 425° 218°
8 450° 232°
9 475° 246°

It sounds to me like the oven thermostat is faulty, as this is what controls the temperature.
If you can , get a thermometer and place it in the oven and check the temp against the seting with the thermometer, if as you say it is 100 degrees above setting , then you need to replace the thermostat.

Most likely you have a bad thermostat.
Oven repair in Mission Viejo, CA
Appliance repair in Mission Viejo and Orange County, CA

If you are monitoring the oven temperature with an analog style thermometer you may actually be ok, but seeing the thermometers slow reaction time.
Some newer ovens actually do pre-heat beep prior to reaching temp in an effort to conserve eelectricity.
A good home test is to try your oven at several different temps to see if there is any variation in internal temperature. Try it one day on 300, the next at 375 and another day at 450. Give your analog style thermometer time to catch up (usually just about 15 min) and leave it on for a half an hour to see if the temp maintains.
Also Beware of aluminum foil AND THE NEW OVEN LINER MATS as they can alter oven performance bay absorbing and blocking the ovens natural heat radiation and convection air flow currents that happen in all ovens, not just convection models.
Another interesting fact is that older ovens used a thermostat in them that operated much like your thermometer and was very slow to react making the oven typically about 100 degrees hotter than where the customer set it. This was just a fact of life before the advent of electronic temperature management and became noticeable when people began trying to bake older "hand me down" recipes in the newer ovens with less than stellar results.
You can mimic the older oven performance by preheating the oven about a hundred degrees higher than called for and after it reaches temp, re-set it to the correct temperature and put the items in right away.
If you find your temperatures fluctuating ask your servicer to replace first the temp sensor and as a last resort the control board as this part is usually pretty expensive.
Good Luck with this!

Hello- The background panel is screened with gradients, but is expensive and hard to find. The easiest solution is to purchase an oven thermometer and allow to set for about 20 min at an indicated 350deg. Make sure that dial knob is tight and mark with a line at that point. Use a fine point sharpy, and clean surface before marking. Turn off, let cool, and turn to the new 350 mark and see if temp comes up and matches 350deg on thermometer. If it works ok, try marking 325, 375, 400, etc. ---Yes,  Clockwise is up in temp. Let me know if this info helps.   Have more if needed-  Thanks - Ed

If it is convection bake you have to turn the temp down (25 degrees or so) that is listed in the recipe.

The broil setting is not used to bake because it operates differently from the bake setting. So, don't go to the broil setting at all.
The element for bake is under the floor of the oven. You have not indicated whether it is gas or electric, but both of these can have issues with the burner or element.

If you are baking on the correct setting and the oven is heating from the bottom, not the top, this can be caused by hot spots that are created by elements/burners.

to check, look under the floor of the oven. Turn the bake heat up to high and see if the flame is uneven. For an electric oven, the color of the element should be evenly red, no brown spots.

If you are using an oven thermometer, move the thermometer to the spot in the oven that seems to be burning things the most.

You need a new t-stat on this unit/ YOu can call us with the brand and model number and we can get the proper parts for you. Hope this helps you out! Thanks, Ella!~ :>)