Hi neuroza
Is your plane made from sign material such as COROPLAST like those shown further down this page under S.P.A.D. Aircraft - Coroplast design? That is what I build and fly. Are you familiar with the SPAD website
http://spadworld.net/forum/ ? That is where many of the SPAD designers/builders discuus this topic. I usually have the center of the tires even with the leading edge of the wing, with tail raised to flying level position. About 1/2 inch back is probably good also.