I found the calculator pretty precise when I launched several frames for an animation. Not 100% precise but never that far away.
Though, for still images it’s more difficult to estimate since for instance ceiling is always easy, floor more complicated etc…
I render locally at 1/2 resolution, so double the resolution on Rebus should be no more, if not less than, 4 times my local time.
lot if wrong infos here
for a DR job we cut the image into regions and render each region by one PC
that means we load and prepare the render 100 times - that time is NOT to pay
we charge CPU usage not PC time so loading happen with 1cpu core only therefore you pay only 1 core !
the popup about the “more costs” just means the loading overhead (1 core time)
we like to avoid users sending 10min renders in distributed cos the overhead is too much in that case
the less rendertime the image has the more effect the overdead has
for a 20 hour render the overhead is not noticable
imagen you send a 10 sec render distributed
load and prepare need 1minute for that job
1min 3% CPU usage + 0.01sec render 99% CPU usage = 3% CPU time you pay * 100 nodes !
insteat one PC do the same render in 1min10sec for 3% CPU
that what the popup mean
but soon things change 
I make a new post about Progressive render at Rebus.