My suggestion is, which ever model program you use and what unit of measurement. Make you models are real scale. The biggest cause for error are the compounding scaling of objects, causes all sorts of headaches and increased light mapping times.
From My usage of all the Products from Classic thru Reloaded to GameGuru now, and funny it is this way, that using Feet and inches for your modeled scenes is the least hassle. The core of the engine seems to like this best. Cant explain why, but after converting 100's of models from 100's of sources for people, that this holds very true, and I have very little problems getting anything into GameGuru this way. Get the object, scale it to proper size, reset transforms, then keep static or rig and animate.
There are a few scripts in the script board that can be combined to make a GPS and range finder and dist calculation utility for test games. They are handy and some are very accurate for this.
The grid is on 100 units which is 254 cm or 8'4".
The scripting ranges, seems to be in inches for some reason but are a little bigger. And I have not quite figured it out. Just know by usage now what I want. Angles are angle for entry and converted to a finer measurement inside the engine for use.
Should be some more chiming in on this.
Intel(R) Core(TM)2 Quad CPU Q6600 @ 2.40GHz, 2400 Mhz, 4 Core(s), 4 Logical Processor(s), 8gb RAM, Nvidia gtx660, Windows 7 Pro 64bit