To answer the first question, we need to look at the factors that influence R. Aside from what the others have said, there is also the companies financial situation and overall business goal. If it is a startup with lots of debt then the R value will be much higher since time is critical factor that will determine if the company stays afloat but if this is simply another launched product that is expected to have stable sales within the foreseeable future then R may be very small. In fact R may actually be calculated using the equation above with projected gains of the product if you are only concerned with getting your money back in lets say 1 to 5 years.
One problem with this is that unless this is used retrospectively or with some historical data at least(in many cases its not), all the components are dynamic changing the factors that affect it completely(which is usually the case and why R is studied so extensively). If the total initial investment needed is not known or changes throughout the project due to murphy or scope creep then all the previous projection change since the time need to get your initial investment will increase if the initial R remains constant. Defining what the goal of the project is, in more concrete or realistic terms than maximize profit, will help set the scale for determining R. The factors that can affect R can be time for ROI, market(gain per unit time), resources available(investment), cost incurred per unit time(used to set a threshold for minimum income per unit time to stay within the goal), overall business model(expansion vs. optimizing and maximizing profit), risk, and business condition/future plans.