Generally the MINIMUM buy in for a ring game is 20X the BB, while the MAX buy in is 100X. 30X usually makes most people comfortable. So divide your buy ins by 30 to get the BB amount. $50/30 = $1.67. So round that down to $1.50, and make the SB $0.75. If you give people chips in $0.25 denominations it will go easily enough. Here's how I would set things up, going by buy in / BB / SB$50 / 1.5 / 0.75$20 / 0.75 / 0.50$5 / 0.10 / 0.05