I play 15 lbs. balls and have a 15 lbs. polyester ball which I mainly use for 8 or 10 pin leaves, 3-10 baby splits and odd leaves (I am a right-handed stroker).
All balls are drilled fingertip, but I do not play my spare ball with any hook, just with a completely broken-back wrist and thumb downward, making the ball skip. It is a remnant from conventional grip times, and it works fine for me. I can play very exact this way, just changing pressure of my fingertips upon the release to adjust by 1 to 3 boards, depending on line and lane condition. Ball goes down the lane almost straight and ends up like a full roller. Only on dry backends the ball will start to make a slight leftward hook, but very controllable.
From my experience a difference between strike and spare ball is counter-productive. For some time I used a 14lbs. ball for spares, and it was not very exact - I threw it halfway down the lane! A friend of mine, playing 15 lbs. too, made similar experiments, and he did much the same and got back to his 15 lbs. polyester ball - itt was even worse because he puts more muscle into his swing.
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DizzyFugu --- Reporting from Germany
"All that we see or seem, is but a dream within a dream..." - Edgar Allen Poe