There must be many variables from the WG's side, some more constant than others. She may (exceptionally) have a fairly decent attitude; yet on a particular day when one sees her, when her intention might be to give an excellent service to a decent customer, she has had a late night the night before, whether from her own silliness or external events, and has an upcoming appointment before which she's done her 'best' and squeezed you in.
What I mean is, we've all had our days at work, whatever work that is, when however hard-working and committed we are that the occasional day goes better or worse than intended. That's more or less common to everyone in life. Now most prossies are not even that committed in that sense, they work for themselves and can take days off when they want. So although you might as a regular feel you deserve an amazing time the reality might work out differently.
I don't think it's worth expending a lot of effort trying to predict a prossie's performance. They're not the Post Office. The only way to go is to treat each encounter on its own merits. Even reviews of encounters. They are what they are. There is mostly no "big picture" of any accuracy, just many odd pieces of information that improve the chances.
I try to sense her attitude, and by that I mean not her pre-defined attitude, which is a mindset and usually negative: but instead where she's coming from at that moment. For the majority, which work with conveyor-belt mentality, I give up after a few minutes and simply try gently to direct the punt to get maximum satisfaction (not hard when her brain's not really in gear, even if the results are fairly limited).
Occasionally a prossie will have a few brain cells left over, not many but a few, that are in reasonable semblance of working order and not involved with the menial task of getting paid, making you come, and closing the door behind you. It's like the faint glimmer of a genuine smile.
Occasionally there is slight sense of genuine human interaction, but rare and nearly always fleeting. It may only last a few seconds and as they say, for everthing else there's Mastercard (or in punting terms, hard cash for whatever you can negotiate off her lying likes list!)