Water level in bowl too low

Hello, W/D, toilet expert, here.
The tank has a water level line cast into the inside back of the tank. That's where your water level should ideally be. Now, there are a variety of water level controllers, so we'll hit on the major ones. But first, a little nomenclature, so you'll know what it isn't. If you remove the tank lid, you'll notice a tube in the middle that's open, and usually has a smaller tube inserted or attached to it. This is the overflow tube, and feeds water to the rim of the bowl. At the base of the overflow tube is the flapper, and it controls the majority of the flushing power. The flapper is operated by a chain connected to the flush lever. When the flapper is raised (flushed), the water level drops. On the left side of the tank is the water fill valve, The water coming from the house via the tubing under the tank goes into this valve assembly. Part of this assembly is the all important float.
Older ones have the float at the end of an arm, As the water level falls, the float falls and operates the water inlet valve at the other end of the float, on the water fill valve. As the level rises in the tank, the float rises with the level and shuts off the water when it gets to it's pre-set level. This type is adjusted by positioning the float either by the screw adjustments on each end of the float arm, or sometimes by bending the float arm.
The newer types have the float either built in, or the float surrounds the fill valve. The one that surrounds the fill valve looks like a very fat donut and is very similar to the older float arm type, just more compact. The water level is set by adjusting the linkage between the float and the water valve at the top of the water control valve.
The newer newer type has all of the floating mechanism built in. But cleverly, there is an external adjustment for raising and lowering the water level. On the side of the tube, there are slots and raised dots on the lower tube. The upper tube has slots on the inside of it. There is a white locking tab between the two tubes. Here's how this one works: (Turn your water off first for adjusting this type). The tube is in two parts, an upper and lower tube. They are held in place by the locking tab, and by the slots cast into the two tubes (they intermesh) To adjust this type, you have to raise the lower portion of the locking tab to raise it off of the locking dots, and than revove it. Then, you push down slightly and twist the upper section to unlock it. Set the level higher or lower, twist the two parts back together, and reinsert the locking tab. The aim is to set the water level at the level mark on the tank. This type can be tricky; you'd do best to turn the water off to adjust this type, and turn it back on once you have locked it back in place.
Best regards, W/D

I had the same problem with the cinnamaron toilet. Water level in bowl was too low And had serious splashback every flush. I decided to look up the installaion manual since the contractor installed for me and I had a feeling maybe something was missed. PROBLEM SOLVED by making sure the white tube that fills the toilet is inserted into the overflow tube at the top right in the middle as per the kholer diagrams. It was not inserted in there and instead it was filling the tank directly. Once I corrected that then the bowl filled the the level as it should be (no more poo smears in the tank) and no more splashback! Try that out.