We make quite a few things from Acrylic and buy ours from Lucite who's Perspex brand acrylic is some of the best around. Over the festive period we had some quality issues with a couple of our products and are trying to make sure that every case we send out is perfect in every way. We put a lot into their design despite most of our cases being derivatives and we really take care over their manufacture. We want you to get a perfect product.
We were surprised when checking our Perspex sheets to find a 10% difference in thickness over a 3mm A4 sized sheet (well 300 x 200mm as we sell here) so we contacted the lovely folk at Perspex to see if this was normal. They replied with their usual speed that this is in fact quite normal and provided us with their spec sheet that made for interesting reading (well interesting for us but we are massive Perspex geeks!). It states that 3mm cast acrylic (cast is the good stuff) can have a deviation in width of -/+ 0.7mm. Wow so it can be as thin as 2.3mm and as thick as 3.7mm (for those of you not familiar with how + and - work...). This means that in building these cases we're going to have to carry several design files to make sure there is a push fit on the majority of parts to make the case feel sturdy. We'll also be grading our acrylic before cutting with the thickest being used for flat designs like our multiholders.
If you want to read about Perspex specifications too we've put the datasheet online here.