Anyway, to address the original question in the general sense, not to the specific case:
What is being asked, cannot be done accurately.
First, bodyfat cannot be accurately measured by any means other than DEXA, which few have access to.
While skinfold for example generally does track change pretty well, the absolute values are often obviously not right, with 2-point error being entirely likely at lower bf values and 3 or 4 points entirely possible at “where are the cuts?” bodyfat values.
So that is a possible error of 4 lb or so, when lean, or maybe 8 lb or so when not.
Next, individuals will vary greatly on the point at which further loss of bodyfat starts resulting in substantial loss of lean mass, and in the extent of that loss.
So, let’s say there’s 6 lb error, and we don’t know which way, in the estimate of LBM. There may, if fat enough to have no cuts, and lacking past experience to judge in the individual case, easily be a 10 lb error in guessing how much LBM will be lost in getting to contest condition.
Now we’re up to 16 lb possible error – and not even as a max, just an entirely likely error.
And we haven’t even looked into guessing how much water weight must be dropped.
The ONLY accurate answer to the question is going and doing it, or having done it a number of times, having the experience to judge how it is likely to turn out this time based on previous results and whatever changes have been made since.