Marc, you are probably going to get mixed reaction from
members here.
I have one, used it thrice, and it sits sadly in the drawer.
It may work OK on the Roos, I bought it for Travancore
and for that, completely useless.
The Roos have quite a strong watermark.
With Roos and KG5 sideface, I drop the stamp face down and dry
on a jet black mouse pad, 80% of the time you can
get the wmk, if it is a toughie, (and it has a hinge)
I drop it backside down on a light dish of water,
as soon as I can spot the wmk, it's outa there.
The real ornery I soak (not mint stamps) and if
you cannot read the watermark then, you're done for.
I was about to spent $200 on a Stanley Gibbons
wmk detector, but was advised by a dealer, they are not much chop.
The real difficulty with the roos lies in the broad and narrow
crown, once you have that sussed, it's a march.
That's just one fellows opinion

No doubt there will be other advice forthcoming.