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You have to multiply the number of rails by 29.
An easy way of doing this is to multiply by 30 and then subtract the number of rails.
Examples:
2 x 29 is the same as 2 x 30 - 2
3 x 29 is the same as 3 x 30 - 3
4 x 29 is the same as 4 x 30 - 4