When creatng a hairpiece mold or template, I think the hairline should be drawn a 1/4 inch behind where you want the hair to appear to come from the scalp. You want hair to appear at the hairline, you do not want the base to appear at the hairline. Please inlcude your opinion.

The  hair foundation is almost invisible, and with their tab material to hide the  foundation and  adhere it to the scalp, no one will notice,
The ventillating  process can begin about 1/16 of an inch from the  front of the  foundation.
If you maintain your hair piece keeping it flat and slightly  stretched ,
then  when you attach it it lays flat, and is unseen.
It is impressive to see the hair apparently coming out of the scalp, very professional
But for the  noob, it will take a few wigs before you are familiar with the  cements.

The other  method is to simply ventelate the hair so it falls forward  , covering up the  foundation material. This works  quite well. You can pull your wig out of the laundry basket and it will look  good.

Another trick to getting a good looking hairline
is to use  bleach
The hair has a loop / knot wher it attaches  to the base, if you bleach the  knot so it is lighter  colored and cant be seen the,  hairline looks much better.

the lace is a  fine knit base
there are even finer knit bases that are more undetectable

Keeping the hairline thinner is better than too thick

AS you get older the hairline recedes a bit.
If you are older it looks fake to have a low  thick hair line