Hi: The formular for the offset is: the outside diameter of the bushing minus the size of the bit used, divided by two. ie the off set with the 3/4" bushing and 1/4" bit would be 1/4". 3/4 -1/4= 1/2 divided by 2 = 1/4. The offset of the 1/2" bushing and the 1/4" bit would be 1/8". I assume that you have a template of the heart shape that you want to use. that template must be used to cut both the the material you want to remove and the material you want to inlay. The material to be removed must be done with the large bushing, and the inlay cut with a smaller bushing, both must be cut at the exact same depth. If you use a 1" bushing and a 1/2" bit the offset would be 1/4" to cut the material to be removed for the inlay. the router base must span the cut out portion of the material. The change to a 3/4" bushing and a 1/4" bit and cut the material to be inlayed keeping the bushing tight to the template.
Hope this helps you out.. Woodnut65