To find the developed area of flow inside the tube we use the empirical relation (X = 0.05 ×D ×Re ). Can we use this relation to find the position of the developed flow inside the spiral tube?
IMO, yes you can if the effect of spiral geometry to the flow is negligible. Another thing is the flow regime should be laminar since that equation is for laminar flow.
The relation, X = 0.05 ×D ×Re is applicable for flow in straight tubes in the determination of entry length. For flow in spiral tube, the flow can no more be considered laminar.