The reason why your getting undercut and other defects on the toes, is the fact you are A. not holding the rod long enough, B. your arc length is possible to long.
While welding vert Fillets its VERY important you keep a very very tight arc,
(if you get a stick rod tun your juice up)
On the 1st pass dont weave just "motor" up the seem, ensuring you dont leave any form of slag inclusion, 2nd pass dont use the have moon method or you will find you will have excess metal dangling in the middle with no where to go. Instead move from right too let or left to right (depending what you prefer)
get a stable arc and weave while keeping a tight arc...
about 150 amps
dont weave drag the rod, ensuring you dont have any slag inclusion (if you do turn the juice up)
maintain an angle of approx 45 degrees on your 1st pass. drop the angle for your 2nd pass ensuring that you are catching the between half lef length on your 1st pass, (slag inclusion guess what turn your juice up)
3rd pass you want to be catching the top half of the 1st weld and ctatching the top top of the 2nd pass if you have slag inclusion ("Dont turn juice up" change your angles and move quicker)