Rob , take Mark's FTP test over 40 min , check his lactate he had there. Move him down so he has only 4 mmol lactate ( if you can do that ) and now check his wattage. Now take Dr. Coggan's scale and zoning sheet.
( Zone 4 threshold which would be , where the FTP 100 % is located. Now compare the wattage he can push by having 4 mmol in the blood and what he pushes, where he was really on the FTP test.
2 and 4 mmol ( I remember somebody telling me , that we use 1970 east german ideas , Mader) I found that very interesting , as we are far away from the idea, that lactate with absolute numbers actually are usefull for any thing but using lactate strips. As much as we are far off the idea , that lactate threshold is still a term used. ( see Robergs , Brooks, Gladden , Kratz and more ).
Does step length will change the wattage and HR values by 2 or 4 mmol , Try it out and you will be surprised.