Basically while you have exogenous androgens in your system then your own natural test production will be suppressed (through the negative feedback loop)
so you need everything cleared
It takes 5 half lives to clear a compound to an insignificant level
Top of my head I think deca is around 12 days half life and test e 8. However individually we do metabolise compounds at different rates so there is some variance.
To accommodate that my rule of thumb is to start after 4 standard half lives.
so 48 days after last deca shot and 32 after last test shot.
hope that helps