During a trip to vegas this last summer, I was scouting a pit looking for a good table. I watched two guys buy in for 2500 each. One of them looked like a complete dirt bag. One was dressed nice. They flat bet and were watching the cards like hawks.
When the count went positive or the cut card that they were looking at came out their bets jumped considerably. No heat whatsoever.
Was watching a table at another casino, jumped in with multiple spots at over 1 k each.. Won some coin and walked away.. No heat. We were there for a half an hour with a PB that liked to ask a lot of questions.
Try a $10 - $120 spread and I think you will be fine.