Can't help: I'm biased!
But let me try....
C=PI*D right? (Even Evo's can accept this - but I would place money on it ;^)
OD = 10 cubits
ID = OD - 2HB
therefore: IC = PI*ID
A question: how many HB's in a CUBIT???
On my measuring stick (arm) it's about 4.5 inches or 4 per cubit. If someone had a skinnier hand it might be slightly less. Something like 4 inches (which is what the standard handbreadth is for measuring horses) or about: 4.5/cubit.
Using the 4" to 18" ratio, you end up with an inside circumference of 30.0195422 cubits or a margin of error of slightly less than 2/100 of a cubit or .36 of an inch.