SurfGF's definitely useful in the excrement/ventilator interface. If you've not got enough gas (bailout), or if it's getting very rough so you need to stop your deco short, or if there's a real emergency... If SurfGF says you're below 100, then you can choose to go.
On the other hand there's the GF99 which I still don't understand! OK, it's the current percentage of ambient pressure to M-value. But it's most definitely a number that requires brain cells to use it.
I find the SurfGF is useful pretty much only when I'm at my final stop and waiting for SurfGF to drop before I make my final ascent to the surface.
GF99 doesn't require any more brain cells to use, I don't think. GF99 is what I would look at during an ascent TO my final stop. Basically, you don't want GF99 to ever exceed whatever you have chosen as your GF Hi setting. It's that simple. If you're doing deco stops, then it gets more nuanced by factoring in your GF Lo setting. But, for NDL dives, it's simple keep GF99 at or below your GF Hi setting.
If you do everything exactly by the book, ascending at 30 fpm, ascending exactly when the computer says you can, etc.., then when you get to the surface, your GF99, SurfGF, and your GF Hi will all be the same number.
What GF99 allows you to do is, in an emergency, you can decide you want to use a higher GF Hi. Say, you're diving GF30/70, but you have an emergency and want to ascend as fast as you'd be allowed with a GF Hi setting of 95. You can use GF99 to do that without actually changing your computer. Just watch GF99 as you go up and don't let it go higher than 95.
Tech note: What I just described will not give you EXACTLY the same ascent as changing your computer to GF Hi of 95. It will be very close. The difference is that if you change your computer, it will let you ascend to a 10' increment that keeps your GF99 below 95. I.e. if you are at 30' and your GF99 would exceed 95 at 25', then it will hold you at 30' until you can ascend to 20' without having your GF99 exceed 95.
If, instead, you ride the GF99 setting up, keeping it at or just below 95 the whole way then, in the previous example, you would ascend above 30' sooner than in the previous example. But, only by a short amount of time.
Lastly, even this explanation is somewhat simplified, as it still does not factor in GF Lo. But, for NDL dives it is (as far as I know) correct.