I have the same type lavender,, I cut it down 1/3 at the beginning of every
spring and scratch in epsom salt into the soil surrounding it, and water it in a little,, When you cut it down 1/3 be sure NOT to cut in the the woody growths as that is it's main life line!
Thru out the
spring and summer where ever it looked 'leggy' I cut those leggy growths down 1/3 as well, and gave it another dose of epsom salt in the mid summer, approx 1/3 cup scratched into the soil,