Well Chris,
Depending on your biological composition this value could fluctuate daily. In order to further analyze this issue you have presented, I will need more information.
I need your exact weight, what day of the week will you be diving, what do you plan on eating the night before and the morning of, are any 'movements' planned in between this time?
Also, how much does the backplate weigh..I need specifics..manufacturer/year made etc....also for your wet suit..who made it, how many dives have you had on it, have you peed in it?
It would also be beneficial if I know what kind of weights you are using.. a 5lb weight from X company is not the same as a 5lb weight from Y company.
Also, what is the fabric of your weight belt and who makes it and what year.
Once you answer all of these questions I will further be able to help you calculate and present you with an answer.
Thanks,