I find it plenty accurate enough to deal in 10s or 5s. But I would add them all up first, and then round up or down the total - so only rounding once.
The rule for rounding to 10s is ..5 and above round up. ..4 and below round down.
So if a meal total was 34g, it would be rounded down to 30g
But if it was 35g it would be rounded up to 40g.
That obviously makes quite a difference, which is why its better not to round all the components, because the multiple roundings might all go in the same direction.
🙂
If you are rounding to 5s, it would be 3 and 8 up, 2 and 7 down
So if a meal totalled
42 would become 40 (down) but
43 would become 45 (up)
and 57 would round (down) to 55
while 58 would round (up) to 60.
Does that make sense?