stickies-v gave where in the code it's computed.

Basically, it does as would be expected to find the percent change of a quantity:

1. Milliseconds elapsed between first and sample blocks:
   Δt = t_FirstBlock - t_SampleBlock

2. How many milliseconds faster the sample block processed than the first block:
   Δprogress = t_FirstBlockProcess - t_SampleBlockProces

3. Gives:
   (% progress per hour) = Δprogress / (Δt × 1000 × 3600),

where (1000 × 3600) = milliseconds in 1 hour.

^{Actually, Δt and Δprogress are both negative numbers, but their ratio in step 3 above is positive.}

stickies-v pointed out in a comment that the code calculating the progress increase per hour is in ModalOverlay::tipUpdate().

Basically, it does as would be expected to find the percent change of a quantity:

1. Milliseconds elapsed between first and sample blocks:
   Δt = t_FirstBlock - t_SampleBlock

2. How many milliseconds faster the sample block processed than the first block:
   Δprogress = t_FirstBlockProcess - t_SampleBlockProcess

3. Gives:
   (% progress per hour) = Δprogress / (Δt × 1000 × 3600),

where (1000 × 3600) = milliseconds in 1 hour.

^{Actually, Δt and Δprogress are both negative numbers, but their ratio in step 3 above is positive.}