compression ratio formula and opinions

So I sold for a friend a pair of low-compression U heads (iron). They are stamped 10.5 and have an enormous chamber (the deepest U chamber i’ve ever seen). Heads were taken from a sidecar U bike, never been milled down. He called me today asking what compression ratio for these heads. My calculations led me to 4.5:1 for a standard 73.79” displacement.
I used the formula:
Combustion Chamber(or 10.5) = displacement of 1 cylinder(or 36.895)/Comp. Ratio – 1
My questions are:
Is that the correct formula?
What should he expect from this compression ratio for a solo bike? Too slow?

Looking for some opinions on this. Thanks.

..

Thanks Panic
Glad to know my math was right.
Also thanks for your suggestions.
I'll talk to him about the mods you listed.
Best regards.
Helsingborg.