Yeah, that is a problem that I encounter with vacuum all the time. The problem is not with the ControlLogix, but with how floating point numbers are represented in any computer. The IEEE-754 float provides for a 23 bit mantissa, which gives you precision of 1 out to 1.7E7 counts. But when you mix big numbers and little numbers, the little numbers get lost in the precision. As your number gets larger, the precision of digits after the decimal place shrinks.
First question I need to ask though is what vacuum transducer are you using? Most get around this resolution problem by using a log linear output, which requires a little more math than just simple scaling.
Second, lets talk about units. One torr is 1mm of mercury, and sea level atmospheric pressure is 760 torr. A micron is 1 micron or mercury, or one millitorr, (1E-3 torr). So, if you want to scale 800 torr to micons, then that is 800000 mirons, not 800. If all you want is microns and your aren't going below 1E-3 torr, then use a DINT for scaling and scale 0 to 800000 (but only if your transmitter is linear).