Let's stop and try to understand the reason for the safety stop.
On a no stop dive, the theory is that the diver should be able to go directly to the surface at any time, assuming a safe ascent rate. On a decompression dive, a stop is required. Decompression algorithms try to predict the difference, but decompression algorithms cannot accurately identify what is actually happening in our bodies, so we introduce fudge factors to add a layer of safety. That is why PADI came up with the oxymoronic "required optional safety stop" for dives on the edges of no decompression limits. They couldn't think of a different term--"fudge factor" was not acceptable. They recommended safety stops for other dives, with no clear cutoff for when they are not needed.
So what does a safety stop do? You have a variety of tissues in your body, and they on-gas and off-gas at different rates. When you start your ascent, the fastest tissues start to off-gas, and as you ascend, all of the faster tissues begin off-gassing. But your slower tissues will still take on nitrogen, even during the ascent. The slowest tissues will still be on-gassing until you get out of the water.
Off-gassing happens when a tissue is supersaturated. For each tissue, there is a safe amount of supersaturation that can be tolerated. Research indicates that when a diver reaches safety stop depth and stays long enough for the fastest tissues to off-gas to that pressure, then the saturated tissues can go safely to the surface, and the tissues still on-gassing cannot possibly reach an unsafe level.
With the Bulmann algorithm, a diver should be able to surface safely with a SurfGF of 100. If you set the SurfGF to 85 and ascend without a safety stop, you have added a level of safety. If you wait in a safety stop, you have added to that level. It stands to reason that if you arrive at the safety stop depth with a surfGF of 60, you should be pretty darn safe to go to the surface.
