I was looking at the setup for a Rainshadow ISP843-3, using a Shimano Sahara 2000, 6 or 8 pound test and starting with a Fuji BYAG25 for the first guide. It looks to me like my first guide will only be 17" from the real face. Does this sound okay? I thought that 19 to 23 would be more the likely range. If it matters any, the choke will be on the ferrul to the third section which is about 2/3 of the way down the rod.

Maybe and maybe not. Although, a size 20 guide is more than sufficient for such light line.

Why not take it out and cast it and see what you think. Then set it up again wtih the size 20 butt guide and try that one.

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If I remember right my Shimano's spool diameter is 42 mm so I use a BYAG 20, instead this will put you in the distances you mentioned from the reel.

Will the 20 be large enough if I occassionally use 10 lb. line on inlets at the coast?

Yes. Height is generally more important than ring size.

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I also just because of gravity I set the path to go into the center of my rings. With Fuji Concept guides.