I never add sinew to the last 6" of each limb tip anyway. That is definately not necessary and will add unnecessary physical weight to the limb tips without any benefit. On a "regular" bow, be it straight, reflexed or recurve I think 65" is to long for a 28" draw to get the full potential from the sinew. 56" or less would be more effective. IMO Maybe with a Mollie type bow where 1/3 of the limb or more is non working it might be effective on a longer bow.
I'm not saying not to do it. That is your call but for me, if I go to the trouble of adding sinew to a bow I want to get every bit of potential out of it.
I agree with the good points you and everyone else on here has made about sinew bows and length, which should be pointed out, and I do realize that this is the general consenses, for good reason of course. However, just to look at things from a different angle, I have sinew backed bows 60" and longer and had good results, compared to how they shot before, (a couple of them were shootable bows before sinewing). Considerable poundage, after retillering, was only a couple pounds I believe, until months after finishing the bow, when the sinew had fully cured and gained a little weight. What I do when sinewing, is either put the bow on a 2 x 4 and bind at the handle, than raise the limbs by wedging things under them, or string backwards and twist up the string with a dowel that has 2 hooks on it to hold the string, until I have as much reflex as I want. If I wanted to sinew a 65" bow (yes it is long, but lets say I did, I have did it before,
) I would do about 5" reflex to start. Lay down a layer of sinew, than raise the reflex an inch or so. Than immediately lay down another layer, raise a bit, than lay down another. Doing that I believe squeeze the sinew together and keep it from lifting when drying. So I would end up with about 8" or more reflex at the end. More if the wood can take it, like osage. With a bow this long, 8" reflex is maybe the equivalent of a 54" or so bow being reflexed 4" or so when sinewed, the way I look at it. On a 65" bow, I might tiller the outer limbs to be stiff, and keep the inner limbs doing most of the work, going for a sort of holmegard tiller, and keep the sinewing on the inner limbs only. But a full bend or even d bow would be easier on set. Either way it would be better to leave the last 6" or so naked.
You could always pike it down if you wanna too?