Do you have a recommended feedline length for EFHW antennas?

No, You can use any length of 50 Ohms Coaxial cables such as RG8x, RG8, RG213, LMR-400, DXE-400Max or similar. The feedline length does not change the electrical length of the antenna nor change the resonant points of the antenna. The minimum SWR points remain on the same frequency regardless of feedline length.