There's a simple penetration calculator at
this location, but this is primarily for determining armor penetration at two periodic ranges and one slope angle in degrees. This can be used in reverse to determine armor thickness, say by plugging in a number for penetration at a given slope and reducing the sample armor penetration until the armor at vertical equals the SPMBT stat for the armor. At that point, the sample armor thickness should equal the actual armor thickness sans slope. For example, say you're trying to find actual thickness of SPMBT armor value 9 at 30 degrees from vertical. Plug in 9 for Penetration, 30 for Angle, and you should get 11. Now reduce the 9 for Penetration in increments until the 11 result becomes a 9. I got 7 for Penetration equals 9 result. This translates to 7 units of armor thickness sloped at 30 degrees from vertical equals 9 units of vertical armor equivalent. To be more exact, 7.25 units at 30deg equals 9.00 units vertical. I don't vouch for the calculator's accuracy, especially at high angles of slope or longer range; and the method is a bit crude, but it comes pretty close without manually applying Pythagorean Theorem (3-4-5 triangle would only give thickness at 30 or 60 degrees slope anyway, and doesn't take penetration dynamics into consideration...wonder if the calculator does?). Note, this calculator works offline, so you can use a saved copy of the page.
If the link above doesn't work, the URL is
http://www.irational.org/tttp/TTTP/idiots/jv_armpn.html
Basileus