I have an MFJ-259 you are welcome to borrow. I can vouch for the advice of the other replies. I have an MFJ screwdriver antenna that I have used portable with 8 radials off a DX Engineering radial plate. I cut mine for a little over 22″ and was hitting central Europe with my Kenwood TS-140s and 100 watts when I used it on the Cape. Sea water was a great help, I’m sure. I also have 20 radials installed in my yard for an LDG s9 31 foot collapsible antenna. 4 of the radials are the suggested 28’10” the others are around 22’8″. Used another DX engineering radial plate. I used the calculations for the shorter ones from one of the articles attached to an earlier reply. Had good results with the set up. As has been stated, it’s a matter of try this, then that until you get either frustrated or good results. For me this is the “fun” (i.e, satisfaction) in amateur radio.