Hi got a MILD worked 351 Cleveland Running 10:1 compression with a 1.1mm head gasket. I know compression be to high for boost (b&m 144 supercharger). Anyone got a rough calculation as to what my compression be around if i ran a 3.2 decompression plate

How big is the cam your using now?????

You haven't given us much to go on but I get about 8.4:1...which is too low even for a lousy 144 blower.

Mate, decompression plates are a pain in the a***! You will need longer head studs (custom), new longer pushrods, good chance your exhaust system will also need to be modified as it mooves up with the heads, and you will need spacers between the heads and inlet manifold to get it to line up correctly, and spacers at the fron and rear of the manifold, where it meets the block. A lot of pissing around that can be avoided by just fitting a different set of pistons. There are JE/SRP pistons to suit your combo available off the shelf for less that $1K.
Decompression plates are for Sigmas, not Clevelands.
Good luck.

On big bore engines it tends to promote detonation also..
Spend the time and $$ on GOOD tune etc..
If it was a turbo ?? That would be different!!
Sensible boost and tune you'll be fine!!
With low comp you loose heaps of throttle response!!