Problems with creating a flexible week forecast

I am currently building an expected payments receipt forecast based on terms of 42 days. The product we are selling is essentially a holiday, so I need to factor in when the receipt would be expected 42 days (and want this to be flexible, adjustable) before the customer arrival date.

I have broken my forecast periods into months, unfortunately unable to do it in weeks as the data is not available.

I currently use the 42 days in fraction form to represent a month. (42/365 * 12 = 1.38) and hopefully use this to drive the month in which the due amount would fall.
Obviously, the immediate problem I am experiencing is that using this, the formula (in cells c15 to n26) will only pick up greater than or less than a whole number. so by design the 1.38 will default to 2.

This isnt good enough for my calcs. Any ideas on how I would factor in the fraction?

One way I thought of would be take the amount and then multiple by a fraction and add on to the balance due i.e. 1 month + 0.38 of the month.
But is there an easier way?

Probably need to look at the attached to understand as I dont think ive explained it very well!

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