The drawback of lack of global reference structure in Decoin may also be a benefit because it makes the coin completely decentralized. And the number of decoins mined at any given moment can be estimated with a simple formula:
m = T(ct/ht) / (c0/h0)
m = number of coins left to mine
T = total number of coins available
ht = hashing rate at the time of measurement
ct = coins mined per hour at the time of measurement
h0 = hashing rate for measurement of initial mining
c0 = coins mined per hour for initial mining
Initial mining means mining from scratch, such as choosing a new and unused MutableData type for the mining to measure c0.
The estimate can be made more precise by taking into account actual operations such as reads and writes done on the SAFE network for the measurements of c0 and ct.
And this is a trustless estimate in the sense that anybody can do the measurement. Several independent measurements can be done at a given moment in time and the median of those measurements will give a good estimate.