How about this in O29:
=LET(period, 5,
value, I14:W14,
flag, I23:W23,
startvalue, FILTER(value,value<>""),
start, FILTER(COLUMN(value),value<>""),
writtenoff, startvalue/period*(COLUMN()-start),
IF(O23="",
"",
startvalue-writtenoff))
If you would drag this down/left/right, this would also take into account the 2 values for the convertor belts, for instance.