THank you I understand completely what you mean about their strategies.
For something that seems so easy I’m not sure why I’m having such trouble. I did give up on prism in the end and used a trial of Sorenson Squeeze (my employer will pay for whatever I choose however haven’t found the right program yet). I liked this one and found a ‘limit bitrate to size’ (or something similar) Sorenson ‘help’ told me this was where to do it, but If I chose 50000 it would end up at 53000 or so , so I’d do it again and guess asmaller amount , so 39500 or so.
I think I’ll use your calculation as explained. Thanks for that. I’ve probably held up this ‘suggestions’ thread enough.
Thanks again & to all who’ve replied.