I’ve never done Lamarck, which was a suggestion by Coach Chad on an early podcast: Lamarck and FTP - #10 by Nigel_Doyle
My go-to has always been to do 30-70 minutes at threshold, without breaks and with a negative split where I typically reach my threshold HR after 10-15 minutes. My original threshold heart rate was based on a Friel 30 minute test and refined on similar long(er) efforts. Essentially a Coggan performance test or more recently a Kolie Moore style test.