300 is the correct setting yes..
There is another version, its WinMX 3.54 beta 4, the added features are not likely to be very different for this issue and if your on 98/ME you are probably just as well off with the 3.53 version you have.
If your on XP/2k then give the beta a try..
Just click on the Archive button at the top of this page and find the WinMX> Beta > Beta 4
Replace the winmx.exe with the new one and restart WinMX...