The signif() function in R is designed for this purpose, but you want more control over rounding behavior, especially when dealing with ties (e.g., numbers ending in 5). Here's how you can modify your function:

MPV_Round_1 <- function(x) {
  # Handle rounding to 3 significant figures
  posneg <- sign(x)                     # Preserve the sign of the number
  abs_x <- abs(x)                       # Get absolute value
  
  # Calculate significant figures based on magnitude
  rounded <- ifelse(abs_x > 0, signif(abs_x, digits = 3), 0)
  rounded * posneg                      # Restore original sign
}

# Sample data
dat_merge <- data.frame(MPV = c(0.000600, 0.0055, 12.3456, 123.456, 999.999, -0.00456))

How do I add this change back to my original data set called dat_merge?

# Apply the rounding function
dat_merge$MPV_Round <- MPV_Round_1(dat_merge$MPV) # use the function on the column of dat_merge

# View results
print(dat_merge)