OP, I hope you get the answer you seek from RH Designs but I do wonder if RH will be able to say anymore than has been said.
Until I looked at the video I had no idea how it worked but it seems that while 10 secs is the default time setting it is possible to move both up and down from 10 secs in 1/6th f stops so doesn't that give the combination of actual time and f stops and thus cover what you want to do?.
Presumably when a new neg is projected onto the easel you, like me or anyone else need to do a test strip so you start at the default of 10 secs and move the dial in fstops both ways so you end up strips that are say 3 x 1/6ths stop either side of 10 or it might be that you know from experience at your chosen fstop in the enlarger lens of say f8 that you need only one/ two stops down and 3 stops up.
What you end up with is a range of exposure in 1/6th stops from which you can see those parts of the print that might need 3x16th stops of a burn or 1-3 stops of a dodgeIt is from this test strip with say 5-8 x 6th stops different exposure that you work our your dodge and burn times
In other words you never need to do what you are trying to do i.e. you never need to combine 4 secs of a non f stop time with an f stop time.
If 4 secs lies in between the nearest 2 fstops then as the presenter of the video says it is irrelevant as you will not be able to see the effect of this marginal difference between 4 secs and the next fstop higher or lower, given that 1/6th of a stop is the best the human eye can discern
Does this timer do what you expected it to do and does it represent value for money compared to other timers be they RH Designs or other makes is a question only you can answer.
Anyway here for what it is worth is the video. which may be the one to which you refer anyway in which case I can only hope that my explanation of my newcomer's view on how it achieves what you need or as close to it as makes no difference is of some value:
https://rhdesigns.co.uk/product/rh-designs-timer-3/
pentaxuser