Having a "speed floor" and I like that term a lot, and calculating an ETA based on a trip planning speed demands quite a bit of effort to maintain. Especially on a 3 or 4 (or more) day passage. If you say "I want to arrive at least two hours prior to dark in 3 days from now you've just put your self on a schedule. Granted, it reasonable to want that to happen but how do you make it happen? You either have to accept the fact that you're on a schedule and "charge the batteries" accordingly or understand and not mind standing off shore until the sun comes up on an arrival day that's later then you had planned. If I
I'm going from one anchorage to another that's a day sail away then I'll put a speed floor on it to calculate an ETA to arrive the two hours prior to dark. If it's a three day crossing then we leave when it's comfortable in the morning usually under power to top off the batteries after a night at anchor. Then we sail when it's nice and motor when it's not and deal with an ETA when we close in on our destination so we arrive in daylight. My point is that an ETAs and speed floors are only required at the end of a trip. The rest of the time I'm just trying to keep my VMG as high as I can by whatever means that keeps the trip fun.