It sounds like you’re using them right, are you zeroing them before you start (not applicable to vernier calipers)? how accurate of a model are you trying to create? The inside jaws are good at measuring parallel surfaces and large holes, but because of the slight offset of the jaws they’re less accurate when you try to measure small holes. To find the center of a servo mounting hole I wouldn’t trust an inside jaw measurement, I would take outside jaw measurements of the overall servo width, then measurements from each side of the servo to the closest edge of the hole, and subtract these from the overall width (I hope that made sense). Or I would just go online and find a schematic.
If I needed to know the size of a small hole to great precision I would find the best fit wire-gauge drill bit (if you have a set, they step in size by a few thousandths of an inch) and go off the known size of it, or just measure it with the inside jaws. The accuracy of the inside jaws doesn’t drop with smaller measurements, as they overlap and are completely parallel. This is way overkill for anything made out of plastic.
By the way, are you using vernier or dial calipers? I lost all patience for analog measuring when I discovered you can get really decent off-brand digital readout calipers (english and metric in one tool, oh yeah!) on e-bay or from local Harbor Freight stores for under $20. Is there one near you? By really decent I mean gets within 0.001" of measurements taken with a $100+ pair of Mitutoyo digital calipers every time!
I would stay away from the plastic (“compostie”) ones, but I use these 6" calipers every day ($20, I got them on sale for $15). I like the thumb-wheel for small motions, but I have friends who love the slightly cheaper 4" model for it’s pocket-size. I’d rather have two 6" pairs, one for home and one for work!