Banana grooves..

I would suspect that the groove location isn't as critical as having it there in the first place. Probably 1/32" give or take would be fine keeping in mind that the grooves must intersect the oil holes and taper to the edge of the rocker width without going past. Shouldn't be too hard to make a tracing from a grooved shaft and add the grooves to a non-grooved shaft with a grinder. Really it's not too cosmic, what you want is a channel with oil under pressure the width of the rocker's thrust face on the shaft