If you think it's a solid cam, start small at like .016" and .018". If it runs like the timing is retarded, open it up to maybe .018" and .020". People have been varying valve lash on both solid AND hydraulic lifters for YEARS to get different results. There's no hard and fast rule that says you have to even run what the cam card says. Since every engine is different, they will likely give optimal power with different lash settings, so people have fooled around with it for decades. Just find where it runs good and leave it alone. Anybody says any different ain't doin nuthin but flappin their gums.
Oh....and some solid lifters DO have that little clip or wire, so that's not always the best way to tell. Read this: