i do sometimes but i dont always know how much to put in ill test this
Try filling the empty cart until the liquid's about a quarter-inch from the top, then (slowly) inserting the rolled-up PTB, then topping it off until the drops stop disappearing into the folds, and it looks like one more drop will overflow.
That might be too much, but it won't be a huge problem (especially if you don't drip on the atty before putting the cart on).
Also, when you start losing vapor, try holding it cart-side up and tapping the LED end a few times on a semi-hard surface (like your knee), or grabbing the cart-atty connection and flicking the other end like an old-fashioned thermometer (but be careful not to let the batt/atty fly off the cart).
I generally get at least a few more puffs by "refreshing" my 510 with tapping or the thermometer shake (I've also seen someone refer to it as the "510 waggle").