You have to scrape the Turcite to fit the bed after it's glued in. That's the thing that matters most. Where that winds up, by itself, doesn't matter much. However, if the apron is bolted to the saddle as it usually is, the angle across the bed will also affect the angle of the apron and the spacing to the leadscrew, feed rods, etc.
You might assume, and this is truly just an assumption, that the builder originally made the crosslide ways parallel to the two flats i see on the outside edge of the bed adjacent to the tailstock ways. If we take that as true, then you would make an initial fit of Turcite to the bed, then measure the distance from the crosslide ways down to the flats, then scrape the Turcite further as required to make the measurements equal front and back.