Ok, so hopefully the table below explains the issue - the 38.00 in column Q is the problem. The issue seems to occur if I have a none on the hour start time with and try to add 30 minutes:
(HOUR(P2+TIME(0,30,0))+MINUTE(P2+TIME(0,30,0)))
<tbody>
L
Start
M
Finish
N
(Deduct 30' Lunch)
O...