A couple of suggestions as it'll be your first visit . . . .
Arrange to take a couple of mates along for an afternoon session - dress it up as your 18th jolly which started at lunchtime and will continue well into the evening until you all fall down. In the afternoons, between 2.30 and 5.00 it won't be too crowded, and all of the real degenerates will have gone back to work.
Flat bet at table mins. Assuming a £3.00 min, £150 will give you fifty bets. On balance of probability, playing for a couple of hours you won't lose it all. You never know, the God of Standard Deviation may smile on you and you might actually end up in front with enough to fund some of the evening jollies?
Don't worry about penetration. True, it does have a profound impact on whether a game is beatable or not, but it won't be as if you can just go and find another table with another dealer if it's not up to much. There'll be plenty of time to be more fussy about such things later on.
Enjoy your 18th, have fun and let us know how you get on with the counting.
If you're serious about making money from playing cards then, as I've already said, learn to play poker. In the UK, with a combination of European NHC rules, crowded tables and a modest spread (I play 1-8 as that's all I can afford) for me BJ is a breakeven game at best. It's fun to play and the counting thing adds something, but I'm under no illusions about it's potential (or lack of one) as a money spinner.
Out of interest, what books have you read?
Newb99