Try:
=(INDEX($T$1:$T$6,MATCH(TRUE,INDEX($S$1:$S$6>=N1,0),0))-INDEX($T$1:$T$6,MATCH(2,INDEX(1/($S$1:$S$6<=N1),0))))/50*((N1-INDEX($S$1:$S$6,MATCH(2,INDEX(1/($S$1:$S$6<=N1),0))))/100)+INDEX($T$1:$T$6,MATCH(2,INDEX(1/($S$1:$S$6<=N1),0)))
where N1 contains the input value (82500)
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