Why does lightoom 6 not keep my custom crop size

Question

Why does lightoom 6 not keep my custom crop size. I want to print A1 (841mmx594m) size so I set an equivalent custom aspect ratio (8.41 x 5.94) and that worked okay. But then I realized I needed a 5mm per side bleed (51mmx604mm) so I changed aspect ratio to 8.51 x 604, but Lightroom keeps changing it to 7 x 5. Now whilst very similar 7x 5 is not the same ratio as 8.51 x 6.04, i.e 851/7 = 121.57, whereas 604/5 = 120.8, so if I multiple 7 and 5 by the lower ratio I get 845.6 x 604 so Ive lost 5.5mm from the landscape length

Why does it do this ?

Edit I wonder if it just accurate to two decimal places and doesn't round up, so these are the dimensions

841/594 = 1.4158 851/604 = 1.4089 7/5 = 1.40

and as you can see the last two are the same to two decimal places if don't round up, but they are different if I do round up.

If I set the aspect to 1 x 1.41 then Lightroom doesn't change it to 7x5, and I think this is close enough as 1 x 1.41 is the same as 851.64 x 604

Answer 1

This has been a known problem in Lightroom for over 2 years, with no response from Adobe.

I'm not sure what the precision is, but if you're too close to a standard aspect ratio it just uses the standard ratio. I needed to do a 13x10 crop and LR refused to store it as a custom ratio, always choosing 8½x11 instead. Curiously, 8½x11 is 1.29412 so 2 decimal places should have been enough to differentiate between the two. It's not clear what's going on under the covers.

According to one comment on the Adobe forum from 2 years ago, older versions (LR3 and before) don't exhibit this behavior.