The SCHAWPF is an awesome fly to fish and tie. There are so possibilities for combinations to use .

AS for your fly, I believe you assessment of the fly is probably correct, maybe a thinner body and a bit more hackle. Having said that, I agree with Jim, it will probably catch some fish Since you said you have never tried anything on a size 18, I would tie some in 14's and 16's and once you feel comfortable with the results then move to an 18. Thats how I do it when tying, work the larger sizes and progress to the smaller sizes.

Keep up the good work and let us know how your flies turn out.

As a disclaimer....I am not that great at tying, but as long as my flies catch fishm then I am not too concerned .

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Take care everyone and cya around. Mark