initial point of contact shown schematically as 46 with the lug l to the outer periphery of the guide 40 should exceed the thickness of the casing to be milled stated differently dimension represented between the arrows 48 should exceed the thickness of the casing to be milled the dimension 48 represents the amount of expected horizontal movement of the guide 40 as the wear pattern illustrated by dashed line 44 is accomplished during the milling operation it should be noted that the window milling operation using the lug l as above described allows for successful lateral deflection of the guide 40 away from the whipstock 20 and toward the casing 18 omitted from fig5 for clarity the operator at the surface curtails the milling operation with the mill 12 has advanced a predetermined amount that amount is a distance generally about 3 feet which is sufficiently smaller than the initial gap between the cutters 42 and the top end 50 of the whipstock 20 an opening such as 52 can effectively be used after milling for a retrieval operation with known 34 fishing 34 tools such as shown in u s pat no 5 341 873 such openings 52 or similar features to facilitate retrieval are preserved and not milled over as with prior designs a lug l of the same design.