Contract payments where the last payment is different

I am trying to make quotes for my orthodontic office which will automatically post the figures for me. I usually just send these to my daughter and she sends them back working but this time we are stumped.

Due to the %discounts the figures don't work out correctly and the last payment is slightly more or less by a few cents, I am looking for the formula which will figure this correctly
EX:
Contract $4140.70

Down payment 25%
=E4*0.25
$1035.18

17 monthly payments of $???$
with one final payment which should be slightly different

I had been using =(E4-D9)/18 for the payments but I can't make that last one calculate to be correct no matter what. I thought it might be the rounding of 2 decimal points??

So a contract of $4140.70
less down payment of 25% $1035.175 rounded to $1035.18
remaining balance $3105.52 over 18 months = payments of 172.528 or $172.52
with one final payment of $172.51
This final payment calculation is where I'm stumped!!!
Any help would be greatly appreciated I have enclosed a portion of the spreadsheet

THANK YOU!!!