You've got the right idea. I will say that, if you intend to sail by magnetic compass, you can round to the nearest degree. Fractions of degrees are pretty meaningless since you can't sail that accurately anyway.
So, your heading would be 137 degrees magnetic. But, that is only a small part of the picture.
You can ignore tidal currents. Where I live, these can readily exceed 5 kts (or considerably more). But, where you live, you can ignore them. Wind-driven currents and seiches are not negligible. Folks with experience on the Great Lakes can better advise you on the best way to predict and compensate for those. (I have no experience on the Great Lakes.)
But, winds have a complicated effect on the heading you ultimately need. When winds produce a current, that is easy enough to compensate for. You just need to know how long you will be in that current. With that information, you know how far in the direction of the current the boat will be translated down-current. From that, you can determine what heading you need to sail.
Put differently, let's say that you want to sail directly east-to-west and there is a current travelling directly south at 1 kt. You calculate that it should take you 5 hours to reach your destination. If you sail directly west, you would then arrive 5 nm south of your intended destination because the current will have translated you 5 nm south. Right now, the water that will be at your destination is 5 nm north of your destination. You need to sail toward that water because, 5 hours from now, that water will be right at your destination.
The problem is that now your heading is different, your trip is longer, and your speed is different than it would have been if you had been able to sail directly toward your destination. There are ways to mathematically account for all of that, but none of them are worth the effort because your speed is not that predictable and currents will change over time and distance.
Additionally, even ignoring currents, your boat has a certain amount of leeway. This is the tendency of your boat to side-slip away from the wind a bit. This varies considerably with the kind of boat, the wind speed, the wind angle, wave action, etc. It is best not to try to predict it with any precision. Just know that it exists and is not negligible.
All this to say that, yes, your destination is 137 magnetic, but if you sail that heading, even if you could sail it accurately, you will almost certainly miss your destination. At that destination, being off just 2 degrees will make you miss the mark by a mile. And, if you can sail a 17-foot sailboat with a magnetic compass within 2 degrees of your desired heading for a trip that will likely take something like 8 hours, then you are superhuman. You would do quite well to sail within 5 degrees, which would put you within 2.5 miles of your destination.
The good news is that none of this matters much. To make that trip in your boat, you need to be out when conditions are clear and winds are moderate. So, you should be able to spot landmarks the whole time. A useful skill to develop would be to take advantage of this fact to hone your triangulation skills. You should be able to shoot an azimuth to the CN Tower and another to Burlington Canal Bridge. Maybe Old Fort Niagara? With those two readings, you can pretty well pinpoint your location. You use periodic checks to adjust your heading.
So, you initially head out at 137 degrees. After an hour, you see that you are a mile north of where you expected to be. So, you adjust your heading a bit south and check again in an hour. If you have been sailing accurately at 137 degrees, that also tells you the combined effects of leeway and currents that you experienced that hour, which might inform how you sail for the next hour.
My first "real" sailing trip was from Seattle to the Canadian border. It was the first time I had ever sailed on saltwater, much less navigated. It was a 22-foot boat. I was a fairly competent sailor and did have experience as an airplane pilot. So, I very carefully plotted out each leg with exact headings and estimated times, based on hourly forecasts. Once I got through the locks and hit salt water, I never looked at the plan even once. I had underestimated how easy it was to simply see where I was going, or see well enough to get me closer. And, I overestimated how accurately I could sail a heading, in particular because winds/traffic/conditions often forced me to sail on a different heading than my destination.
