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I think SMR is all over the map and try to use completed sales and VCP. When it comes to half grades I just compare the specimen in question to the surrounding graded examples on VCP or anywhere I can get scans. If it compares favorably to the next grade up, looking just like or exceeding those examples in eye appeal, then I think paying anything under what the grade up costs is a great value.

For example, I've seen 8.5s of cards that look just like 9s to my eye-- and if I have to pay way higher than an 8, but still less than the 9, I'm cool with that. Conversely, if a .5 card doesn't look much better than the usual examples at the next lower whole grade, I can't see spending more just to have a card with a higher sticker/grade. All about the card, end of the day.

A card is worth what you're willing to pay for it...there is no secret formula!

And to add on to that, I personally think the half grades are a bit redundant. We have always paid more for the best looking examples within a grade level. I know I have and I think most collectors do.
I wouldn't put much stock in the SMR. I like PSA but that guide isn't close on many issues, VCP or CardTarget would be much more accurate, imo. And I am fully aware even those sales can be manipulated too...but they are still better than some made up guide price with little basis in the real world. I compared my T205 Cobb to the beautiful one last night in auction and I think it's a toss up on which I think is more aesthetically pleasing. But one is a 6 and one is a 7, so there is probably a 7k difference because of the flip...

Sometimes good sometimes bad. I've had half grades sell for the SMR of the grade below and above. Recently got murdered on a 7.5 Rose... Previous sale was $1750~, mine sold for $1150~... HUGE variance all in that single half point.

Buy the card not the holder. I would NEVER auction or consign for auction a card with a half grade that I may have paid up for. Re-sub it if you think it should be the next grade or put it in as BUY IT NOW. These half grades are way too volatile to take the risk. It seems to be that if half grade cards are well centered they will carry the premium well IMO.

Jason

A general way to price it as a potential buyer is as good looking example for the lower full grade (in the grader's opinion). And especially good looking examples for a whole grade have always sold for a premium.

As already said, look at the card and decide what you think. It was just a little silly when many PSA half grades initially sold for more than the higher whole grade. Bidder exuberance for something new, or they thought PSA was too shy to bump it another whole grade.

Also, PSA hasn't always judged on overall aesthetics. For example, N172 Old Judges have gotten high grades even when the images were light. Most N172 collectors desire a good image. A T206 with especially radiant colors will sell for more, even when the grade is technically the same. I don't know if PSA now uses aesthetics as part of the reason to give a .5 bump. Maybe they do.

PSA might rightfully respond that they grade physical condition, and are not art critics.

When I sell cards, I usually price the half grades around 1/3 higher than the lower full grade. For example, if I'm selling a PSA 6 at $75 and a PSA 7 card from the same set at $125, I usually price the PSA 6.5 at $90.

I always think that theoretically, a half grade ought to sell for halfway between the price of the higher grade and the lower grade. But prices have been all over the map. When PSA first started with half grades, they were selling close to the level of the higher grade. But that has since calmed down, I think.