Anybody know the math here?

I am putting together my list of rides for my Knighthood attempt and was curious if someone knows the math on the impact of reduced intensity on the IF & TSS of any given ride.

Example: Ride X has a TSS of 100 and an IF of 0.90 when done at 100% intensity. If I am doing the ride at a lower intensity, say 75%, what happens to those two numbers? Is there a way to recalculate the two numbers?

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Here is the formula:
TSS = t * IF * IF * 100 where t is the duration in hours.
(Source: Estimating Training Stress Score (TSS) | TrainingPeaks).
I substituted for NP, using the definition IF = NP/FTP.

In your original example you would be riding for about an hour and a quarter.
Lowering your IF to .75 would give you (for the same time), a TSS of about 69.

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IF scales linearly. So a 0.9 IF workout would come in at 0.675 at 75%.

TSS scales by the square of the adjustment (as formula above). So 100 TSS at 100% would become 56 at 75%.

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