What should be optimal gas limit and gas price for 80 addresses airdrop?

Question

This is my airdrop function:

mapping (address => bool) public airdrops;
function airdropTokens(address[] _recipient, uint256[] _tokens) public onlyOwner{
uint airdropped;
for(uint256 i = 0; i< _recipient.length; i++)
{
      airdrops[_recipient[i]] = true;
      require(token.transferFrom(msg.sender, _recipient[i], _tokens[i] * decimalFactor));
      airdropped = airdropped.add(_tokens[i] * decimalFactor);
}

}

What should be optimal gas limit and gas price for this? I initially used, gasPrice = 50000000000 and gasLimit = 4500000, which works out at $47.27. Is this normal? I managed to successfully send a batch of 20 addresses at 3.5 GWEI / 2500000 Gas limit see: airdrop to 20 addresses transaction

But for other batch of 80 addresses, it failed at 4 GWEI / 4500000 Gas limit, see link here Any ideas on how to manage the gas better? If I was to send out the 350 more batches at the recommended prices, it would cost me around £16k!!

P.S: Currently, each airdrop transaction is sending tokens to 80 addresses at once.

Answer 1

Here are few hints:

1. replace public modifier with external in your contract. Here is why
2. Not used gas (limit) is returned back to you so you may put as much as possible. On time of writing it's around 6-7M.
3. You may put low gas price (e.g. 1 GWEI). However, your transaction will take some time. You may estimate it here: https://ethgasstation.info/