accounting for model scale for proxies and conversions

is there a general rule for accounting for various differences in height/size when proxying or using "counts-as" models?

for example, using the angel of radiance from reaper minis in place of GW's Sanguinor? due to GW's flying pose, the sanguinor is positioned higher than other units of his actual physical size, making him harder to give full cover... the angel of radiance, unless given an elevated base, would be positioned lower to the ground... making it easier to give cover to. the issue really only applies to shooting vs cover, because close combat is determined by base size, but i can foresee the issue coming up on occasion.

There's no official rule. Generally if you're worried about a counts as/proxy you need to s/w the tournament or event organizer to see if they'll allow it.

If if it's a casual game you just need to work out how you'll rule on it with your opponent.

Worse case scenario: buy and base the actual GW model if you're really terrified of someone throwing a fit about it. That way if it's ever called into question, put the actual model down as close as you can and make the call that way.

In most cases you have to be careful, with Sanguinor I wouldn't worry too much. He's about head and shoulders taller than other models, but as far as cover is concerned that's not really a big deal. Since area terrain gives him cover automatically, and intervening models do the same. A quarter of an inch height on a standard infantry (even if he is jump infantry) model will make little to no difference except for line of sight and hiding him. In general though, if you are making a counts as of a model that exists (this applies more to MCs than anything) you want to try to keep them as close to the same size as possible. Tiny Daemon Princes are a little hard to justify ( I'm looking at you Will! >_< )

Generally the "rule of cool" applies. If it looks like your just a gamer fag and putting all your marines on kneeling legs for some advantage you'll be looked down upon. If you have a cool idea and model and paint it and it is a good looking counts as almost no one will have a problem with it. I personally wouldn't have a problem with it. A tad naked for a 2+/3+ though...