I swap/add/subtract something on most weeks and it really depends why I’m swapping as to what I look at.
When subtracting is the goal:
TiZ would be good to look at if I’m swapping out a longer Z4+ for a shorter Z4+ because of time constraints…this typically adds to the IF though.
TSS would be good to look at if I’m adding strength training that day or just need a little lighter activity because of slight fatigue.
When adding is the goal:
I go longer duration of the same or similar workout (or adding some Z2/Z3 to the end) when my legs are feeling good.
I go up in intensity of the same or similar workout when my legs feel great.
As always, this is just what I do…bad or good…and may not work for others.