If you're looking for a meeting place or a good stop around ½ or halfway between San Francisco and Arcata, you can try Redwood Valley, CA, which is a little over 2 hours north of San Francisco, or almost 3 hours southeast of Arcata.
About ¼ or a fourth of the way, you can stop at Windsor, CA. This is a little over 1 hour north of San Francisco.
About ⅓ or a third of the way, you can stop at Ukiah, CA. This is almost 2 hours northwest of San Francisco.
About ⅔ or two thirds of the way, you can stop at Pratt Place, CA. This is about 3 hours northwest of San Francisco, or about 2 hours south of Arcata.
About ¾ or three fourths of the way, you can stop at Phillipsville, CA. This is about 3½ hours northwest of San Francisco, or about 1½ hours south of Arcata.
If you drive straight through without stopping at all, the total distance is 279 miles. The distances shown above don't include local roads to get into each town, so if you're planning a long road trip, make sure you factor in traffic and road conditions, rest stop breaks, and interesting detours along the way. If you're in a rush, you can do the trip with zero stops, and your total travel time would be around 4 hours, 58 minutes.