From the IRS Recordkeeping
To deduct your losses, you must keep an accurate diary or similar record of your gambling winnings and losses and be able to provide receipts, tickets, statements, or other records that show the amount of both your winnings and losses. Refer to Publication 529, Miscellaneous Deductions for more information.
despite the above this is what the PUB says regarding playing slots
These recordkeeping suggestions are intended
as general guidelines to help you establish your
winnings and losses. They aren't all-inclusive.
Your tax liability depends on your particular facts and circumstances
Slot machines. A record of the machine number and
all winnings by date and time the machine was played. It seems winnings and losses on the same day can be netted for the same type of activity. so slot winnings and losses on a day can be netted but if play keno, that win or loss is separate.
so assuming you have the requisite data
you would figure as follows
cash start of + cash put in say 1500
net winnings on days when there are net wins say 6000, total so far 7500
taxes taken out 2000 net so far 5500
cash at the end 100 losses 5400 on days with net losses
put another way your losses came from the following
net cash out 1400
wins net of withholding 4000
losses 5400
