The supernova rate is only estimated/known to a factor of 2-3, so high levels of accuracy are not possible or warranted.
If we assume high mass stars are born uniformly in a disc of radius $r$ light years, then you can work out a "surface density rate" of supernovae of $1/100 \pi r^2$ per square light year, per year (adopting your rate of one per century over the Galaxy).
Then, consider a thin annulus of radius $x$ and width $dx$ around the Earth. The number of supernovae that explode per year will be
$$dN = \frac{2\pi x}{100 \pi r^2} dx$$
and the number that have exploded in the last $x$ years, who's light is yet to reach is just this multiplied by another factor of $x$.
If the Sun was at the centre of the Galaxy the calculation is then simple
$$N = \frac{1}{50r^2}\int_{0}^{r} x^2\ dx = \frac{r}{150},$$
with $r$ measured in light years. If $r\sim 30000$ light years (I think 50,000 is a bit big), then the number is 200.
Unfortunately, that isn't the geometry. Instead of being a circular annulus around the Sun, you have to work with an annulus that is truncated where it reaches the "edge" of the Galactic disc. I may add to this answer later, but I doubt this will change the number above by very much.
