Dynamic MAx formula

You can't assign a formula like that, formulaarray needs r1c1 style references.

Range("T3").formulaarray = "=MAX((r3c3:r10000C3=r[]c3)*(r3c21:r10000c21))"

To make it dynamic:

Sub macro_1
dim LetterCycle, Dayslate
LetterCycle = 3
Dayslate = 21
Range("T3").formulaarray = "=MAX((r3c" & LetterCycle & ":r10000C" & LetterCycle & "=r[]c" & LetterCycle & ")*(r3c" & Dayslate & ":r10000c" & Dayslate & "))"