One very general rule of thumb for all bikes is that at with leg at furthest extension, there just be just a bit of a bend at the knee. This is with leg at "6 o'clock" position for most bikes. (on my recumbent bikes my legs are out in front so it's more like "3 o'clock".)
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Bacchetta Giro A20, RANS V-Rex, RANS Screamer