Remember, stock cables on the Dyna aren't going to be the same length as the stock cables on your '98 softail. +4 on the Dyna which already had 10" apes may not be the same for a softail.
I'm looking to use the same bars on my Heritage. I asked the same question and most guys went +6 on the throttle, idle and brake and +4 on the clutch. I ordered my bars and cables last week, so I can't verify 100% this is correct, but going 4" higher with 4" more pullback from the stock bars I figure +6 should be about right.