If you calculate the average depth over the "slow" section of the ascent you can then use ratio deco to have a decent guess at home much extra deco time your slow ascent added. In this case the average ends up being 170ft ish, 4 extra mins at 150 would be about 4 mins deco but being a little deeper makes for about +5 or 5.5 mins extra.
30ft/min is the buhlmann default ascent rate. Slower than that might seem safer but you're way way below the GF high down there anyway. My preference would be to ascend at the 30ft/min rate knowing I am below GF high (of 75% in my case) and for a chunk of that time below even GF 30low. Why set my GFs at 50/75 if I dont follow that? If I wanted 10/90 or 30/70 or some other ascent in my mind I should pick that and use it. 17ft/min is doing exactly the opposite of what the science is telling us about deep stops and bubble models. I sucked it up in the interest of staying close to my buddy, but its too slow for me.
Here's two dives from class, three screenshots from the first one and two from the second. Using shearwater desktop you can see what your TTS is doing and calculate your ascent rates.
Shallower dive I did the gas switch on the fly, primary reg free flowed and it wasn't pretty. Deeper dive the next day, I had no intentions of switching on the fly - I switched when I hit my stop.
The screen shots show that with all that added complication on the shallow dive - per my computer, I gained two minutes deco - the deeper dive, slightly slower ascent rate, "late" gas switch, I gain 4 minutes of deco.......
Is plus 4 minutes that bad if you don't mind spending the time?
My question revolves around different motives, different diving style. Those were classroom wreck dives, I dive warm water drifting reef dives. We've dove a lot lately in the 100-130' range, plans based on gas and limiting TTS to no more than 10 minutes - our ascents are slow. TTS gets to say 8 minutes, we head shallower gradually and may spend another 40-60 minutes in the 70-40' range. First stops are almost always at 20' by the time you get there. We may carry more gas and extend to 150', similar type profile with the slow ascent - are we really being that bad to ourselves with the slow ascents? GF Lo is 45 now, Hi is 80 but I'm usually out of the water at 75..........