Hi,
we normally use 100ug/ml final concentration of BSA for this, but based on your conditions you have to optimize this.
And also your protein should be purified as much as possible
Not exceed more than 15ul of loading volume, if you increase the loading volume, you are increasing the salt concentration and this will effect on PH and finally it will create problem on running condition. If possible add 1 ul tRNA in addition Heparin sulfate.
good luck