Small 15 (S15) = Small chain ring, 15 rear cluster (middle)
Small 21 (S21) = Small chain ring, 21 rear cluster (one down from easiest)
Large 15 (L15) = Large chain ring, 15 rear cluster (middle)
During 20 Minute test effort use gear selection of choice with cadence of 85-95. Try build in first 2 minutes then apply sustainable effort through to end, re-assess sustainability every 5 minutes.
Record average Heart Rate for the 20 minute effort and multiply by 0.95 for your Lactate Threshold Heart Rate for a 1 hour effort.