Darryl’s Analysis

Environment:
The light situation look adequate for a rubber plant – right in front of these windows will be fine.

Effort:
In terms of watering, the WHEN is correct (when completely dry) but your HOW should not be “pouring a small amount” – this is mostly for future reference since you say you’ve only owned the plant for less than a month.  In general, when it’s time to water, you should evenly moisten all parts of the soil and let excess water drain away.  With a large pot that does not have drainage holes, you should aim to pour in enough water to “fill” roughly a quarter of the total volume of soil – try to evenly distribute the water and pour slowly.

Because your plant is getting adequate light, I suggest you regularly use fertilizer – rubber plants (and most houseplants) benefit from a 3-1-2 fertilizer.  More on fertilizers (with links) here.

Expectations:
I wouldn’t be too concerned about the plant since it’s the lower leaves that are falling off – which is inevitable.  If you manage your soil structure (by occasionally aerating it), you should be able to maintain good oxygen levels in the root zone, which should minimize the appearance of black spots.  Do this by gently loosening the soil with a chopstick prior to watering.  I do this around once a month (not every time I water).

Rubber plants can grow nicely for many years but you should expect their overall shape to change.

Learn my long-term enjoyment approach to houseplant care – learn from my course.