How to make different font sizes on graphs with specific point sizes

I'm trying to get specific font sizes for scatterplots based of Nature guidelines. I'm told to keep all my point sizes for text at 7 so I've used par(ps = 7). However, I need a different font size for the main title. I need 8 point font size. I understand I can use cex.main but I'm so confused about the ratios to get that exact point size. Can someone tell me how to get exactly 8 point font size for the title.

Here is my code for one of the graphs:

par(mfrow=c(2,2), ps = 7)

par(mar=c(6,4,1,0))

#Plot for Confirmed Tick Data
plot(tick1_data$Year, tick1_data$average_cases,
     main = "a",
     cex.main = 1/1.14, ## I'm not sure this is giving me 8 point font size ##
     type = "p",
     pch = 19,
     cex = 2,
     col = '#BF0436',
     xlab = "",
     ylab = "Average Confirmed Cases",
     ylim = c(0,308),
     bty = "n",
     xaxt = "n")
mtext(side = 1, text = "Year", line = 5)
par(bty = "n")
axis(side = 1, at = seq(2006, 2017, by = 1), las = 2)
rect(2004, -10, 2100, 50, border = NA, col = "#d3d3d365")

lines(spline(2005:2017, tick1_data$average_cases, n = 100), col= "#BF0436", lwd = 2)