Lemon Thyme Substitute
Lemon zest and thyme would be a perfect substitute for lemon thyme. Lemon thyme is an entirely different species from regular thyme, but the name is very apropos. Figure maybe 1 part finely grated zest to 3 parts thyme. As with all herbs, the ideal amount is really very subjective. Start with half of what your recipe calls for, taste your dough and guess from there. You won't really know until you taste the baked shortbread, but you won't be terribly off either.

As a general rule of thumb, 3 parts of fresh herbs equals approximately 1 part of the same herb dried (whole leaf). If your dried herb is ground (or rubbed), figure just over half of the dried, whole leaf amount. Know that ground herbs and spices lose flavor quickly (as in they are pretty tasteless after a matter of a few weeks).

It means nothing, but I hate lavender: Is there a typical ratio for Herbes de Provence? :)