I'm not really sure what you're looking for. The code you posted should work, though you probably want to give it a more useful name so you can access it later:
Code:
f.HealthText = f:CreateFontString(nil, "ARTWORK", "GameFontHighlight")
f.HealthText:SetPoint("BOTTOM")
Then whenever you want to make it show some other text, just set its text:
Code:
f.HealthText:SetText("15%")
If your text involves any string.format or concatenation, use :SetFormattedText instead of :SetText to move the formatting C-side:
Code:
local currentHealth, maxHealth = UnitHealth("player"), UnitHealthMax("player")
f.HealthText:SetFormattedText("%d/%d", currentHealth, maxHealth)