bagmaker wrote:

Lets suppose I am on a shallow final glide, 60k out in my 40:1 ship,
cruising at 60 knots. Recent thermals have been at least 5 knots and I
am coming into some big lift. For the discussion rough air Vne is 100
knots.
What strength thermal should I take to increase finishing speed to Vne
and how much (if any) time will this save me?

Assumptions:
1) 60 knots at curent altitude will get you there (just).
2) 60k is 60 nautical miiles(nm)
3) there is no wind - this keeps your example simpler
4) 40:1 is at 60 knots
5) glider sink rate at 100 knots is 6 ft/sec

So you know you will finish in 1 hour at 60 knots, current altitude.
At 100 knots, you will finish in 60nm(1 hr/100 nm) = .6 hr = 36
minutes.
But you need to know how long to climb so you can go 100 knots.
Your current altitude is 60nm/40 = 1.5 nm.
1.5nm(6000 ft/nm) = about 9,000 ft.
36min(60 sec/min) = 2,160 sec
2,160 sec(6 ft/sec) = about 13,000 ft of altitude needed.
This means you need to climb about 4,000 ft.
A 5 knot thermal will give you about 500 ft/min.
So it will take you 4,000ft(1min/500ft) = about 8 minutes to climb.
So total time is 8min + 36min = 44min, or 16 minutes faster than
1hour.

How do I estimate this at the time?

I wouldn't. As others have indicated, use of a flight computer is the
best way to go.

Regards,

-Doug