How to replace main sail halyard advice please.

[sailorman]

Sorry to say this, but the OP shows a worrying lack of imagination and practicality. It is surely not beyond the wit of man to use the existing halyard to pull up a measuring tape, or even a bit of string. Double that length, and add on the requirements at deck level. Job done.

From the OP

[ my wife is pretty handy with a needle and thread and thinks she can sew a new halyard end on to a new one and therefore pull the new one through and into place with out having to take the mast down. I wonder if this is a viable idea or a recipe for disaster and an expensive visit to the shipyard?]

 

[NormanS]

From the OP

[ my wife is pretty handy with a needle and thread and thinks she can sew a new halyard end on to a new one and therefore pull the new one through and into place with out having to take the mast down. I wonder if this is a viable idea or a recipe for disaster and an expensive visit to the shipyard?]

I'm not sure what your point is. The OP was wondering about a "formula" to work out the length of a replacement halyard, and saying that the original builder was defuct, so couldn't be asked. What I'm demonstrating is that it is not difficult to establish the required length.
His wife has worked out a way of joining the new to the old.

 

[Pcd]

Sew my new to old with dental floss and put one wrap of insulating tape over the join. (It's the only sewing I've ever done - zero skill or technique). Yet to fail. Test the bend. Take it slowly and if in doubt stop and start again. I understand the worry when you haven't done it before.

 

[Mandarin331]

Just pulled the topping lift out for a wash in the same way we normally do, sew a loop into the end, pass light blue rope (such as from any builders merchant) through the loop, double it back and pass three times under single strand of the body of the blue rope and tape the end down so it won't snag, then pull gently through, the sewn loop looks like this...
View attachment 48766

I just leave the loops on all year ready for next laying up. I assume this is the same process many of us use.