IMO, often the "line" is not so much of a concern for me WRT the post. looking at many of them the material is often very thick.

but a too short amount of post can damage a seattube if is doesn't span the entire toptube/seattube junction WRT depth. possibly even then.

i'll spare you the embarrassment of explaining how you ended up with a too short seatpost by not asking. BTW, i have a NOS Nehru jacket that's too small for me that i can let you have at cost.

in case you've forgotten or never knew...