You could get away with no offset on the servo arm but there will be slack at max deflections. My buddy did a really cheap fix to the same setup you have (well close they are Nelson horns). He just cut up some Hitech blue anodized alum servo arms (I have like 50 of them) and bolted them to the ends of his non-offset servo arm. He made it so the offset ends are bolted onto the end of the straight arm at right angles. So he basically fabricted his own offset into the servo arm. He all but eliminated the slop and I was pretty impressed with how it looked. I think he had to use about 4 servo arms that he cut the hubs off (or maybe left them on can't remember, you get the idea). He needed two per side for the the offset and two more as cross braces to keep the offset pieces from turning. I will attach a stick sketch I made up in Microsoft Paint. If you like the idea credit goes to Todd (Twistawrecka) on RCU. His were 3/8 but you would need to measure your distance from the control horn flag hole to the hing line and that's the offset you need.

Oh and by the way that's a good question how long the servo arm has to be with the offset, maybe Scott at SWB could answer that. I think it should be 3 inches though, same as control horn.