msatoshi is in millisatoshi precision; it can be a whole number, or a whole number ending in msat or sat, or a number with three decimal places ending in sat, or a number with 1 to 11 decimal places ending in btc.
There are two considerations for how good a route is: how low the fees are, and how long your payment will get stuck in a delayed output if a node goes down during the process. The riskfactor non-negative floating-point field controls this tradeoff; it is the annual cost of your funds being stuck (as a percentage).
For example, if you thought the convenience of keeping your funds liquid (not stuck) was worth 20% per annum interest, riskfactor would be 20.
If you didn't care about risk, riskfactor would be zero.
fromid is the node to start the route from: default is this node.
The fuzzpercent is a non-negative floating-point number, representing a percentage of the actual fee. The fuzzpercent is used to distort computed fees along each channel, to provide some randomization to the route generated. 0.0 means the exact fee of that channel is used, while 100.0 means the fee used might be from 0 to twice the actual fee. The default is 5.0, or up to 5% fee distortion.
exclude is a JSON array of short-channel-id/direction (e.g. [ "564334x877x1/0", "564195x1292x0/1" ]) or node-id which should be excluded from consideration for routing. The default is not to exclude any channels or nodes. Note if the source or destination is excluded, the command result is undefined.
maxhops is the maximum number of channels to return; default is 20.
The formula used is the following approximation:
risk-fee = amount x blocks-timeout x per-block-cost
We are given a riskfactor expressed as a percentage. There are 52596 blocks per year, thus per-block-cost is riskfactor divided by 5,259,600.
The final result is:
risk-fee = amount x blocks-timeout x riskfactor / 5259600
Here are the risk fees in millisatoshis, using various parameters. I assume a channel charges the default of 1000 millisatoshis plus 1 part-per-million. Common to_self_delay values on the network at 14 and 144 blocks.
1 is a conservative value for a stable lightning network with very few failures.
1000 is an aggressive value for trying to minimize timeouts at all costs.
The default for lightning-pay(7) is 10, which starts to become a major factor for larger amounts, and is basically ignored for tiny ones.
The final id will be the destination id given in the input. The difference between the first msatoshi minus the msatoshi given in the input is the fee (assuming the first hop is free). The first delay is the very worst case timeout for the payment failure, in blocks.