Is this advice too late ?
After installing my own IMHO the ratio is neither here nor there. 4:1,
5:1 will do the job, and just ensure that the cement is malleable
enough to 'work with'. However, I would say, make sure you place the
cement to fill the gaps in reverse side of the shower tray. This would
be low and smooth ish in the middle and a highish ridge running around
the outside. That will ensure no movement. I believe that's the desired
result when installing !
Oh and make sure you clean up the cement from the outside edges once it
is in place. This will ensure whatever flooring you choose fits neatly
around the shower base. It also means there's no chiseling after it has
set to remove it, and the potential mishap in hitting the shower tray
HTHs - Oliver.