The number you are entering should be the circumference of the tire, measured in millimeters. 1254 looks pretty small for your wheel. You can get the circumference by rolling the tire one revolution and measuring the distance with a tape measure. I usually lay out the tape, start with the tire valve at zero, roll one revolution, and read the number. Of course this number could be off by a few percent as the tire isn't exactly the same size due to rider weight, air pressure, tire wear, and temperature. If the error is large you need to do some riding on a known distance, like a running track, and then tweak the number up or down slightly until you are happy.
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