against current is no good if one or both of them run fast.
BTW if you are doing a more general exercise in wx, it is good to look for local patterns. It is often with the wx that i before e except after c. Look for broad (seasonal) then shorter (multi-day) patterns. You will find many regularities and a number of irregularities. Lear to 'sense' them as they are rolling over and turn one into another.
Your best sailing weather is often related to such patterns and their movements.
In most cases, what we know from reliable wx sources (e.g. NOAA) is our best choice in making safe sailing decisions. The case is similar to understanding the finance: either apply the broad and general rules or else go full monty and learn the subject thru and thru. As they say 'a little knowledge is a dangerous thing'.