Your existing approach would become:
However it's worth pointing out that each of these SUMPRODUCT is Volatile and each is referencing close on 20000 cells ... that's going to be very (very) bad news for the performance of your file 
If I were to make a suggestion... it would be this:
Calculate the position of the "next low value" in each row on the Target sheet where you have a value in B (eg somewhere on CP8 row 204, 205 etc etc...)
You can then use a less expensive formula approach from your Summary sheet - ie along the lines of your existing LOOKUP approach in column C.
eg
Then
Given the sheet variable nature of your functions you can't (with native formulae) avoid Volatility however when using Volatile approaches it is imperative that the functions used are not memory hungry - ie avoid Volatile Arrays & Sumproducts like the plague.
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