Howe doesn’t have plus-minus tracked in his career totals for the first 13 seasons of his career, which was 826 games. It wouldn’t surprise me to learn that his is actually something like +400.
This is getting off topic, but
here's a thread where HOH tries to estimate Howe's ES goals for vs goals against data for the first 13 years of his career (before plus/minus was tracked).
The data in table equates to approximately +366. Add that to his official +160 from 1960 onwards, and you'd get +526. That would rank him 4th all-time (and 1st all-time among forwards, though he's essentially tied with Gretzky). Does that seem like an unreasonably high result? I don't think so:
First, the official plus/minus data only captured Howe from age 31 onwards (and he went +160 in in 921 games, or +14 per 82 games). Howe was almost certainty a better player in the 1950's (when he won five of his six Art Ross trophies, and four of his six Hart trophies). He unquestionably played on better teams (Detroit led the regular seasons standings seven times in the 1950's; they only finished first, or even second, once in the 1960's). This analysis suggests that Howe would have been about +366 over 846 games (+35 per 82 games). That's a higher rate than the official data, but this would capture his peak years, on a stronger team - it makes sense that the rate would be higher.
Second, nobody should use this to argue that Howe is greater than Gretzky. Keep in mind it took Howe an extra 280 games to reach (essentially) the same plus/minus as #99. Gretzky is ahead on a per game basis (+29 vs +24 per 82 games). At the same time - Howe was by far a better defensive player than Gretzky, and I don't think it's unreasonable to suggest that, in terms of career value, 26 seasons of Howe approaches the impact of 20 seasons of Gretzky. (The plus/minus data mirrors what makes sense in terms of their reputation - Gretzky clearly peaked higher, but Howe was an all-star level contributor for longer).
Last point - look at the all-time leaders in plus/minus. There are exceptions, but majority of them had long (15+ year careers), personally had a reputation of being a good two-way player, and generally played on strong teams. Examples of this include Bobby Clarke, Bryan Trottier, Ray Bourque, Nicklas Lidstrom, Denis Potvin, Larry Robinson, Chris Chelios, Scott Stevens, Al MacInnis, Serge Savard, etc. Howe is a perfect match for the profile of the majority of players above +350.
I know this is off topic, but I agree - Howe's full career plus/minus is almost certainly north of +400, and probably above +500.