I was wondering if there was any way to roughly estimate your compression without using the tester? What info do i need to figure it out. I just need a rough estiment for ordering a carb.
i have a '85 250r, ported and polished, bored .30 over, Weisco piston, FMF pipe.

 

the compression tester won;t give you your compression ratio. You need to know your bore size, stroke, head CC, gasket CC, and dome area of your piston. Has your motor been modified other then a bore job? If not, then figure 6cc for the piston dome, around 4.25cc for the gasket area, I would guess around 26cc for a stock head, but others can probably get you more accurate info on that. With a .030 overbore, your displacement rises from 246cc to 252.34cc. Thats the displacement of the cylinder. Add 26cc for the dome, and 4.25cc for the gasket and you get 282.59CC. now subtract out the 6cc dome of the piston and you get 276.59cc. add this number to your dome volume to get total volume of air and you get 302.59cc. Divide by the dome area and you get your compression ratio:
11.63:1, which seems high, but...
this is just a model to use as an example, to get really accurate you need to know deck height, your true head cc, etc.