Offset IF Function That Checks For Mathematical Errors?

I have an IF Function that I want to use to check mathematical errors between two sheets. For example Sheet2 is where I put all my material prices and Sheet1 cell A1 shows the total amount that is one Sheet2. On Sheet1, I have an IF Function in B1 that does the same calculation A1 does, but will display "ERROR" if A1 does not Match B1.

The IF Function works, my problem is that I want this to work from B1:B100 on Sheet1. But on Sheet2 the next SUM would be from row 42 to row 59.

How can I make and IF Function from B1:B100 on Sheet1 offset every 20 rows on Sheet2

For example:

Sheet1 B1 would be:
Formula:

[Code].....

Sheet1 B2 would be:
Formula:

[Code] ......

I need to do this without having to manually enter each range to sum up.