It usually works ok if you follow the instructions or sim some historical aircraft to get the hang of it but can give strange results.
For Tanks there is the Tanksharp sheet, which I've been going through (v0.6) and is full of calculation errors which I'm slowly correcting. Simple stuff like only accounting for one side of the hull armour, conversion between cm2 and m2 incorrectly. I'll post a corrected sheet eventually.
Good point on the side armor (also applies on the turret armor, at least for 0.71), correcting this is getting my designs much closer to the real world without needing my "structural weight" hack. Still a bit light, it looks like, but much closer.
Does anyone know of either a good website or book one could buy on aircraft that were designed, but never saw service? Especially Polish ones, such as the PZL P.62.
Judging by the picture there and over here: http://www.samoloty.ow.pl/ (though that one's in Polish), the PZL-62 is kind of reminiscent of the middle prototypes of the He-100 with the retractable radiator. The armament's pretty heavy, with a 20mm motor-cannon and 6 or 8 wing mounted 7.92mm MGs, all powered by a Hispano-Suiza 12Z
That shouldn't be too much of a problem for the plane's specifications. You can easily figure out what is what by looking at the figures. I do the same with the information of planes on this board where everything is in Czech.