Probably different for everyone, but...
A dashboard isn't supposed to invent or be the source of business intelligence (opinion), it's job is to support and supplement what's already around in an informative and fresh way. If you are intended to project KPI, look for projections already out there to assist you. Good sources are budget plans, staffing plans, sales look-aheads, etc (accounting, finance, sales, human resources) internally. If you are lucky enough to have leading indicators from external data sources (stock prices, home sales, unemployment numbers, internet clicks, etc), go find that information. Then, look at how that information compares with current trends to see which factors can be used reliably to project things for your company.
I'd probably say it's more of an art than a science, but that doesn't mean science isn't involved...
: )