Perhaps you are not familiar with the evolution of the slowest of the Winchester pistol powders, or else you wouldn't be so anxious to load this.
First in the series was 630. After several years and many reports of blown up revolvers with no logical explanation, generally with loads significantly below what was listed as maximum, Winchester pulled the stuff off the market.
Somewhat later they introduced a replacement called 630P. After several years and many reports of blown up revolvers with no logical explanation, generally with loads significantly below what was listed as maximum, Winchester pulled the stuff off the market. Is this starting to sound familiar?
Finally, after supposedly doing additional development work, Winchester introduced another new ("New"???) powder called 296. After several years and many reports of blown up revolvers with no logical explanation, generally with loads significantly below what was listed as maximum...... (Starting to get the picture?).
They finally did some extensive experimentation trying to determine what was causing guns to blow up when less than maximum loads of 296 were used. I hope you are familiar with the many, many articles in the magazines and denials by both Winchester and Hodgdon (H-110, identical to 296 and manufactured by St. Marks also) that this was even happening, they did finally determine that there were very real, although still not fully understood, situations where a reduced load could and did destroy a revolver in a very obvious over-pressure event.
After three attempts to market what were, at least 3 very similar if not identical powders, under three different names, they finally determined that these should not be downloaded (more than 3% Winchester & 5% Hodgdon) or pressure spikes, pressure excursions, or any of several descriptive phrases which were used, could occur without any logical reason. Basically, factory data was supposed to be used as is, hence the Winchester warning to load EXACTLY as listed.
Are 630, 630P and 296/H-110 the same powder? They are, at least, very similar and from the same manufacturer. Has there been the same experience of reloaders with guns being blown up under unexplainable circumstances by all three? Absolutely YES. Is there factory established maximum data for 630? NO.
I know you are going to load it, and you might not have any problem. I wish you luck. Either that or it still makes good fertilizer just like it did 35 or so years ago when it was discontinued due to the problems which were caused by it.